Which one of the following is not a property of 'graphene'?

Correct — D, It is a wide band-gap semiconductor. Graphene is the exact opposite of a wide-gap material: it is a zero-gap semiconductor, often called a semimetal, because its conduction band and its valence band touch at points in the electronic structure known as the Dirac points. There is no energy gap for an electron to jump, which is precisely why graphene conducts so freely — its resistivity is about 10⁻⁸ ohm-metre, lower than silver's, the lowest of any metal at room temperature, and its electron mobility runs beyond 15,000 cm²V⁻¹s⁻¹. That missing gap is also graphene's commercial problem rather than its selling point: a transistor needs to be switched off as well as on, and a material with no gap will not switch off cleanly, which is why so much research goes into opening a gap in graphene artificially. The other three statements are all true. A sheet of graphene is a single layer of carbon atoms, the thinnest two-dimensional material there is; it absorbs only about 2.3% of the light passing through it, so a layer is roughly 97.7% transparent; and it is among the most conducting materials known.

• (a)It is the thinnest material known so far. — True, so it cannot be the answer to a 'not a property' stem. Graphene is one carbon atom thick — a single sheet of the layers that stack up to make graphite — and is described as the thinnest two-dimensional material there is.
• (b)It is almost completely transparent. — True. A single layer absorbs about 2.3% of light across the visible and infrared range, leaving roughly 97.7% to pass through. This combination of transparency with conductivity is what makes graphene attractive for touch screens and flexible displays.
• (c)It is highly conducting. — True, and it is the direct consequence of the property that makes option (d) false. With no band gap to cross, charge carriers move almost unimpeded; graphene's resistivity is lower than that of silver, the least resistive metal at room temperature.

Graphene is a single, one-atom-thick sheet of carbon atoms arranged in a hexagonal honeycomb — one layer of graphite, isolated and made to stand on its own. Andre Geim and Konstantin Novoselov achieved that at the University of Manchester in 2004 by peeling layers off graphite with adhesive tape, work that won the 2010 Nobel Prize in Physics. The sheet is the strongest material ever tested, with an intrinsic tensile strength of about 130 gigapascals and a stiffness close to a terapascal, and it is at the same time nearly transparent and extraordinarily conducting.

Band gap is the discriminator the examiner has chosen, and it is worth fixing the three cases in one line each. An insulator has a large gap that electrons cannot cross. A semiconductor such as silicon has a small gap, so a modest input of energy pushes electrons across and the material can be switched between conducting and not conducting. Graphene has no gap at all — the bands meet — so it always conducts, which makes it a superb conductor and a poor switch. Everything else in the option list follows from the same structure: a single atomic layer is bound to be thin and to let light through, and the delocalised electrons above and below the sheet are bound to carry current. Only the band-gap claim contradicts what graphene is. Note also that the paper files this item under information technology; the reason is applied rather than academic, since graphene's transparency plus conductivity is what makes it a candidate to replace indium tin oxide in touch screens and displays.

• Graphene is a zero-gap semiconductor, or semimetal, because its conduction and valence bands meet at the Dirac points.
• A single layer absorbs about 2.3% of incident light from the visible into the infrared, so it is close to 97.7% transparent.
• Its resistivity is about 10⁻⁸ ohm-metre, lower than that of silver, and its room-temperature electron mobility exceeds 15,000 cm²V⁻¹s⁻¹.
• Intrinsic tensile strength is about 130 GPa and the Young's modulus is close to 1 TPa, making it the strongest material tested.
• It was isolated and characterised in 2004 by Andre Geim and Konstantin Novoselov at Manchester, who shared the 2010 Nobel Prize in Physics.

• Assuming that because graphene conducts well it must be a good semiconductor; conducting well and switching well are different demands.
• Reading 'thinnest material' as loose praise — it is literally one atom thick, and a real physical claim.
• Confusing graphene with graphite. Graphite is a stack of such sheets held by weak forces, which is why graphite marks paper and graphene does not exist naturally on its own.

Usually as a which-statement-is-not-correct item on graphene's properties, or as a match between carbon allotropes and their distinguishing features.

• practice — not a real PYQ

  Graphene is best described in electronic terms as

  • (a)a wide band-gap insulator
  • (b)a zero band-gap semiconductor
  • (c)a superconductor at room temperature
  • (d)an ionic conductor

  Answer(b) a zero band-gap semiconductor — its valence and conduction bands meet at the Dirac points, which is why it conducts so freely and why it is hard to switch off.

• practice — not a real PYQ

  Graphene was first isolated in 2004 at the University of Manchester by

  • (a)Harold Kroto and Richard Smalley
  • (b)Andre Geim and Konstantin Novoselov
  • (c)Sumio Iijima and Robert Curl
  • (d)C. N. R. Rao and Venkatraman Ramakrishnan

  Answer(b) Andre Geim and Konstantin Novoselov — they peeled single layers from graphite with adhesive tape and shared the 2010 Nobel Prize in Physics for it. Kroto, Curl and Smalley are the fullerene chemists.